1. Field of the Invention
The present invention relates to identifying optical fibers, for example optical fibers in an optical cable. An optical cable can contain a large number of optical fibers, for example several hundred optical fibers in the case of a an optical system transmission cable.
2. Description of the Prior Art
An optical fiber conventionally comprises an optical core, whose function is to transmit and possibly to amplify an optical signal, surrounded by an optical cladding, whose function is to confine the optical signal within the core. The optical fiber is protected by a covering, generally of polymer resin, whose function is firstly to protect the fiber during handling and installation in an optical system and secondly to limit deterioration of the fiber and drift in its optical characteristics over time.
An individual optical fiber placed in an optical cable of a transmission line of a given optical system must be identifiable, either for connection purposes when installing the system or for checking purposes in the event of tests or system reconfiguration. Furthermore, in the case of a dense wavelength division multiplexing (DWDM) transmission optical system, managing the chromatic dispersion of each optical fiber of a transmission line becomes critical and it is important for each fiber of a cable, with its optical dispersion characteristics, to be accurately identified, because connection errors cannot be tolerated in this kind of system.
There is therefore a real need for a fast and certain method of identifying each optical fiber in an optical cable.
One conventional solution to identifying the fibers in a cable, in particular for the purposes of connecting the cable to an access network, is to use a color code. The polymer covering of each fiber is colored, at least in an end portion of the fiber, each color corresponding to a fiber type. The covering can be colored either directly, by adding pigments or dyes to the formula of the resin, or by applying a colored layer to the covering. A dozen colors have been standardized.
This technique has drawbacks, however. The number of colors that can be used to color the resin of the covering is limited and insufficient to identify all the fibers in a cable. Fibers with a colored covering are usually placed in tubes of different colors inside the cable, which increases the number of identifying color combinations. This technique is complicated, however, and liable to manipulation and interpretation errors.
Another prior art technique for identifying optical fibers in a cable adds colored rings to the ends of the fibers. This solution, which is sometimes combined with the previous one, increases the number of identifying color codes, but the number nevertheless remains insufficient. Furthermore, adding a ring around the fiber introduces a mechanical stress that can modify the local optical properties of the fiber, which can be harmful if the transmission windows are very tightly packed.
What is more, none of the prior art solutions for identifying optical fibers in a cable can mark a fiber with an accurate record of the fiber or identify separately different portions of the same fiber. Coloring the covering or applying a ring may indicate the fiber type or state at a given time, but does not indicate any change in the characteristics of the fiber.
Furthermore, color codes cannot prevent errors of interpretation by technicians manipulating the cable, who have to consult correspondence tables to recognize the colored fibers.
An objective of the invention is to address the problems and limitations of the prior art previously referred to.
To this end, the invention proposes to identify a fiber directly in its covering by laser marking a coloring element disposed in the covering. Laser marking has the advantage of inscribing an identifying code on the fiber by localized modification of the coloring properties of the coloring element.
The invention also proposes an easy, reliable and powerful method of identifying optical fibers in a cable.